How to redirect to a specific scene on start-up, but allow redirection afterwards in BGE?
In general I suggest to start the game with the current scene. A dependency to a specific scene will make development much harder as you always need to go through this opening scene. Switching right at the beginning increases load time.

Logic handler

For easier handling I suggest to create a group with one object that performs the switch. This way you can easily create one instance at each single scene without creating it again and again.

Another option is to link an object between the scenes.

Concept

At game start you want to switch to the opening scene when:
- the opening scene was not visited before

You need a sort of store/restore method. 

Storage

As you switch scenes, I suggest to store such information in a Python module. It will survive any scene switch (in difference to object properties). The storage will survive as long as the game session runs.

Code

The code is pretty simple. Please keep in mind it should run in module mode (script mode will not work this way).

caller.py

alreadyCalled = False

def isFirstCall(controller):
    global alreadyCalled

    if alreadyCalled:
        return 

    alreadyCalled = True

    for actuator in controller.actuators:
        controller.activate(actuator)


Setup

Always Sensor -&gt; Python Controller Module: caller.isFirstCall -&gt; Scene Actuator: to opening scene


Remarks

This will work with the first call "caller.py". Any subsequent execution of "isFirstCall" will not activate the actuators even when executed within the same frame.